Background and Indications for Examination
Ultrasound evaluation of the inferior vena cava (IVC) provides rapid, noninvasive assessment of a patient’s hemodynamic status at the bedside. The size of the IVC and its respiratory variability has been shown to correlate with right atrial pressure (RAP) and intravascular volume. These observations are valuable in estimating RAP, detecting changes in intravascular volume, and monitoring a patient’s response to volume resuscitation.
Structurally, the IVC is a thin-walled, highly compliant vessel. Its size and dynamics vary with respiration and changes in intravascular volume. The development of negative intrathoracic pressure during inspiration increases the venous blood return from the extrathoracic veins into the right heart. This leads to an increase in the blood flow through the IVC and a subsequent decrease in its blood volume, resulting in a reduction in intraluminal pressure. These changes decrease the diameter of the IVC during inspiration relative to expiration. These observations are reversed with positive pressure ventilation in which IVC diameter increases during inspiration.
In patients with a low RAP and/or intravascular volume, the IVC size is relatively decreased and its respiratory variability is increased. If RAP is very low, the IVC can collapse completely during spontaneous inspiration. In patients with high RAP and/or intravascular volume, the IVC size is increased and its respiratory variability is decreased. The IVC is very compliant, but its capacity to distend is not unlimited and is restricted by connective tissue in its walls and surrounding structures.
Traditionally, central venous pressure (CVP) and volume status in the acute care setting have been measured by placing a central line. Central lines are invasive, time consuming to insert, and may cause significant complications. Bedside ultrasound has been shown to provide a good estimation of CVP in place of more invasive methods. The clinician can perform serial IVC measurements on an ill patient in order to guide their decision in providing more intravenous fluids or to administer more aggressive medication therapy.
Probe Selection and Technical Considerations
To visualize the IVC, a phased-array (frequency of 2.0–4.0 MHz) or curvilinear probe (frequency of 3.5–5.0 MHz) should be used. These relatively low-frequency probes provide better penetration and visualization of deep structures.
Depth of field should be adjusted to allow complete visualization of the IVC and its entrance into the right atrium. The depth needed will mostly depend on the habitus of the patient. Obese patients will naturally have deeper vessels and therefore require an increased depth setting.
Time-gain compensation or far gain should be adjusted in order to account for loss of signal that can occur in the far field, and to provide a relatively uniform intensity across the entire depth of the image.
Color-flow Doppler is used to detect the presence, magnitude, and direction of blood flow. It is useful in differentiating blood vessels from other structures and artifacts in the abdomen.
M-mode ultrasound detects the motion of structures located along a single axis as they move toward or away from the transducer over time. This method is particularly useful in displaying changes in the IVC that occur during respiration and is the preferred method for quantifying IVC size and its respiratory variability.
Normal Ultrasound Anatomy
The IVC is the largest vein in the body. It drains blood from the lower extremities, abdominal wall, and the visceral structures of the abdomen and pelvis and returns it to the right heart.
The IVC is formed by the union of the common iliac veins at the level of the fifth lumbar vertebrae. It ascends cranially through the abdomen along the right side of the aorta. It passes anterior and to the left of the right kidney where it is joined by the renal veins at the level of the second lumbar vertebrae. It continues its ascent along the left border of the descending part of the duodenum before it passes behind the intestine and enters a groove on the posterior-inferior surface of the liver between the right and caudate lobes (Fig. 8-1). It then passes posterior to the portal vein and is joined by the hepatic veins prior to passing through the diaphragm at the eighth thoracic vertebrae. Its course ultimately ends at the right atrium of the heart into which it drains blood from the lower body.
The IVC can be visualized in a transverse (short-axis) or in a sagittal (long-axis) orientation while scanning the abdomen. Imaging is performed with the patient in a supine position using the liver as an acoustic window. It is recommended to begin with a transverse view, with the probe indicator to the patient’s right side just below the xiphoid process. In this orientation, the vertebral body of the spine is seen as a bright hyperechoic structure with posterior shadowing. The aorta is seen in cross section just anterior to the spine as an anechoic blood-filled circular structure. The IVC is seen adjacent to the aorta on the patient’s right side as an anechoic circular or teardrop structure (see Chap. 5). The image on the screen will depict the IVC on the left side and the aorta on the right. As the IVC is followed toward the heart, the left, middle, and right hepatic veins will be seen to enter the IVC just before it enters the right atrium.
Care should be taken not to confuse the IVC and aorta. The IVC will be thinner-walled, more compressible, and will often have a teardrop shape as opposed to a circular conformation in a transverse view. The IVC will show respiratory variation and may pick up pulsations from the adjacent aorta so “pulsatility” seen on two-dimensional imaging should not be used to differentiate the two. Color-flow Doppler may be used to confirm characteristic arterial flow in the aorta.
After completing the transverse view, a sagittal (longitudinal) view is obtained by rotating the probe 90° clockwise so the indicator is toward the patient’s head. This is also called a subcostal long view and will display the IVC in its long axis, with the left side of the screen toward the head and the right side toward the feet (Fig. 8-2). This image will depict the IVC coursing along the posterior border of the liver, crossing the diaphragm, and draining into the right atrium. It is often recommended that the sonographer obtain a subxiphoid cardiac view first and then rotate the probe 90° into the subcostal long orientation in order to obtain an adequate image.